[doc][manual] Minor fixed and remove last todo-note

• Fix some typos
• 'project onto'
• unified spelling: 'vector field', 'driven-cavity'
• Explain why we need to explicitly give a function range
• Use consistent format for broken code lines
• Use consistent notation for functions/methods: If we talk about the function, just use its name without '()' if we talk about an expression or function call which happens to have an empty argument list, then keep the '()'.